First off i want to say Good Job for choosing to a adopt a dog!!
Combining older dogs and younger dogs is usually not a huge issue. Start by introductions. Have ur dogs meet in a "neutral territory" keeping all the dogs leashed. Let them sniff each other and just get to know each other. Sometimes it does take time, so dont be discouraged. As long as there not fighting then thats a good sign.
Once u get a dog make sure u are doing activities as a pack. Taking all of them for a walk. Feeding them at the same time. There are many things u can do to bring u and ur pack closer.
Depending on what breed u get i dont think u need to be very worried about a new dog trying to run away. Most shelter dogs somehow understand that u have saved there life and tend to be very loyal to there owners. I wouldnt leave a door wide open or anything like that, but i wouldnt be to concerned about this issue.
Let us know when u get ur new dog and post LOTS of pics!!